1. Field of the Invention
The invention relates to a supporting apparatus for supporting a thickening at the Dash neck of a single crystal consisting of semiconductor material in a crystal pulling installation which operates using the Czochralski crucible pulling process, and to a process for producing a single crystal using the supporting apparatus.
2. Background Art
During the Czochralski crystal pulling process, first of all a neck of greatly reduced cross section is grown in order to suppress further growth of crystal defects which are already present in the seed crystal or originate from the seed crystal coming into contact with the melt, such as dislocations in the cylindrical part of the single crystal. The narrowed neck is referred to as the Dash neck.
Following formation of the Dash neck, the parameters of the pulling process are set in such a way that a conical portion is produced, followed by a cylindrical portion with a much larger, and virtually constant diameter. The pulling process is in turn concluded by growth of another conical portion. Usually, only the cylindrical portion is of interest as a product, for example for use in the electronics industry. For economic reasons, the diameter of single crystals, has in the past, been increased in a stepwise fashion. By way of example, silicon single crystals with a diameter of 300 mm are now currently produced on a large industrial scale. At the same time, attempts have been made to produce single crystals with the longest possible cylindrical portion. Overall, this has led to continuously increasing single crystal masses.
The increased crystal size and length have led to the problem that the Dash neck is no longer able to bear the tensile forces associated with the increased mass. The Dash neck is not generally able to reliably support silicon single crystals with a mass of more than about 250 to 300 kg. Consequently, methods and apparatuses have been developed for providing heavy single crystals with additional support during their production. In most processes of this type, the Dash neck is widened at a specific axial position, in such a way as to form a thickening. A supporting apparatus which absorbs some of the tensile force can be positioned at this thickening. Nowadays, a large number of different supporting apparatuses which operate according to this principle are known, cf. for example U.S. Pat. No. 5,879,448 and U.S. Pat. No. 6,077,347. U.S. Pat. No. 5,879,448 describes supporting apparatuses which can move vertically within the crystal pulling installation and can reduce their internal diameter in the manner of tongs. For example, they can grip the crystal after a thickening has been pulled at the crystal. An alternative option consists in a supporting apparatus with a U-shaped bearing surface being pivoted under the thickening after it has been pulled, as disclosed in U.S. Pat. No. 6,077,347.
It has been found, however, that all the known supporting apparatuses generate particles (for example through abrasion), which can lead to the single crystal being contaminated with metals. This is unacceptable given the high purity requirements of the electronics industry. Moreover, particles of metal or silicon oxides (SiOx) can lead to the formation of dislocations during crystal pulling. Condensing silicon monoxide (SiO) can cause jamming in bearings and joints of the supporting apparatus.